The present invention relates to a color selection mask structure for color cathode ray tubes. The invention finds its application in any type of tube comprising a color selection mask and is more particularly adapted to a tube having a mask therein held under tension by a frame to which it is secured.
Conventional cathode ray tubes comprise a color selection mask situated a precise distance from the inside of the glass front face of the tube, on which front face are deposited grids of red, green and blue luminophores so as to form a screen. An electron gun disposed inside the tube, in its rear part, generates three electron beams directed towards the front face. An electromagnetic deflection device, generally disposed outside the tube and close to the electron gun, has the function of deviating the electron beams so as to make them scan the surface of the panel on which the grids of luminophores are disposed. Under the influence of three electron beams each corresponding to a specified primary color, the grids of luminophores allow the reproduction of images on the screen, the mask enabling each specified beam to illuminate only the luminophore of the corresponding color.
The color selection mask must be disposed and held during the operation of the tube in a precise position inside the tube. The mask holding functions are carried out by virtue of a generally very rigid rectangular metal frame to which the mask is conventionally welded. The frame/mask assembly is mounted in the front face of the tube by virtue of suspension means which are usually welded to the frame and cooperate with pegs inserted into the glass constituting the front face of the tube.
The current trend is toward tubes having flatter front faces, with a tendency towards totally flat front faces. To make tubes including a front face of this kind, requires a technology consisting in the use of a flat mask held under tension in at least one direction. Such structures are described, for example, in U.S. Pat. No. 4,827,179.
Since the color selection mask consists of a metal foil of very small thickness, its tensioning can give rise to unwanted phenomenon of setting the mask into vibration while the tube is operating. Under the influence of shock or outside mechanical vibrations, for example acoustic vibrations due to the loudspeakers of the television set into which the tube is inserted, the mask can start to vibrate at its natural resonant frequency. The vibrations of the mask consequently modify the zone of landing of the electron beams on the screen of the tube. The points of impact of each beam are then shifted with respect to the associated luminophore grid, thus creating a decoloration of the image reproduced on the screen.
The above-cited U.S. Pat. No. 4,827,179 proposes the addition of means for damping the vibration of the mask to a face of the mask. However, the damping devices implemented in this patent have a complicated structure which is difficult to implement. It is therefore desirable to have a cathode ray tube comprising a mask structure with simple and inexpensive damping means.
A cathode ray tube, according to the present invention, comprises:
a color selection mask in the form of a substantially rectangular metal foil, adapted so as to be fixed under tension to a support frame and mounted inside the front face of the tube, the mask comprising a central zone drilled with orifices and a peripheral zone disposed between the central zone and the edges of the mask, the mask being able to vibrate independently of the support frame; and
means for damping mask vibrations which are disposed at the said periphery of the mask so as to damp the vibrations in the mask, the damping means being characterized in that they comprise a resonator in the form of a flexible metal strip of which one part is fixed to a surface of the peripheral zone of the mask with two wings extending on either side of this one, central, part.